Method for improving the operation of chloro-alkali diaphragm cells and apparatus therefor



1969 M. P. GROTHEER 3,471,382

METHOD FOR IMPROVING THE OPERATION OF CHLORO-ALKALI DIAPHRAGM CELLS ANDAPPARATUS THEREFOR Filed Dec. 1, 1966 United States Patent 3,471,382METHOD FOR IMPROVING THE OPERATION OF CHLORO-ALKALI DIAPHRAGM CELLS ANDAP- PARATUS THEREFOR Morris P. Grotheer, Lewiston, N.Y., assignor toHooker Chemical Corporation, Niagara Falls, N.Y., a corporation of NewYork Filed Dec. 1, 1966, Ser. No. 598,243 Int. Cl. C0111 1/06; C(llb11/26; B01k 3/00 U.S. Cl. 204-98 8 Claims This invention relates to anapparatus and method for improving the operation of chlor-alkalidiaphragm cells. More particularly, this invention relates to anapparatus and method for increasing the effective life of the diaphragmused in diaphragm type chlor-alkali electrolytic cells and formaintaining an uninterrupted constant flow of electrolyte through theelectrolytic cell while maintaining the current efiiciency of the cell.

Chlor-alkali diaphragm cells are used for the production of largequantities of chlorine, hydrogen and caustic, particularly caustic soda.These basic chemicals are produced by electrolyzing an aqueous solutionof an alkali metal chloride in a cell having a diaphragm separating theanode from the cathode. The diaphragm serves to maintain the chlorineand hydrogen produced by the electrolysis separate from each other. Thediaphragm is of a material having a suflicient porosity to permit thedesired flow of brine from the anolyte compartment to the catholytecompartment while keeping the hydrogen and chlorine gas separate fromeach other.

One of the most suitable diaphragm materials used in chlor-alkali cellsis asbestos. Although other materials such as woven polypropylene,Teflon, and the like synthetics can also be used. However, in theoperation of such electrolytic cells, the diaphragm eventually becomesrestricted by deposition of impurities contained in the brine andsolubilized from the concrete cell structures which are normallyemployed, and thus the flow of electrolyte through the diaphragm isreduced. The height of the brine within the cell increases as thediaphragm loses its permeability thus requiring the lessening of theflow of brine to the electrolytic cell to compensate for the decreasedflow through the diaphragm. Eventually, the diaphragm becomes sorestricted that it is desirable to replace it to maintain a desirablecell efliciency. Normally, as the diaphragm becomes restricted, and thebrine feed rate decreased to avoid overflowing the cell, the efiiciencyof the cell decreases because of the reduced flow and the correspondingincrease in caustic concentration within the catholyte compartment. Theincreased caustic concentration increases the back migration of hydroxylions into the anolyte compartment thereby reducing the anode efiiciencyof the cell.

The need for replacement of the diaphragm normally occurs two or threetimes during the life of the graphite anodes. The replacement of thediaphragm requires the shutdown of the electrolytic cell, thedisassembly thereof, the removal of the cathode section on which thediaphragm is positioned and the replacement of the diaphragm with a newdiaphragm followed by reassembly of the cell. Because of the frequencyof replacing the diaphragm, the

labor involved in disassembling the cell, the production time loss andthe decrease in electrical efliciencies prior to the replacement of thediaphragm, it is particularly desirable to eliminate or lessen thenumber of occasions during which the diaphragm is replaced.

It is an object of this invention to provide a method for maintainingthe current efliciency of chlor-alkali diaphragm cells while extendingthe useful life of the diaphragm. It is another object of this inventionto provide an apparatus particularly suited for maintaining the currentefiiciency of the chlor-alkali diaphragm cells while reducing thefrequency of diaphragm renewal. It is a further object of this inventionto provide a method whereby maintenance and adjustments of brine flowthrough the electrolytic cell can be largely eliminated. These and otherobjects will become apparent to those skilled in the art from thedescription of the invention which follows.

In accordance with the invention, an electrolytic diaphragm cell for theproduction of chlorine and caustic is provided having an anodecompartment and a cathode compartment separated by a porous diaphragm,the improvement comprising feeding brine solution to the anolytecompartment of said cell while feeding a supplemental brine solution tothe catholyte compartment of said cell in excess of the flow through thediaphragm. In a more specific embodiment, the cell is operated byfeeding brine solution to the anolyte compartment of said cell at a ratein excess of the flow through the diaphragm, withdrawing the excess feedsolution from the anolyte compartment, circumventing the diaphragm withsaid excess feed solution and passing said excess solution to thecatholyte compartment of an electrolytic cell. In addition to the methodof operating the electrolytic cell, an apparatus is provided which isparticularly suited for maintaining a constant level within theelectrolytic cell while withdrawing the excess anolyte solution from theanolyte compartment and feeding it to the catholyte compartment of theelectrolytic cell.

The present invention has numerous advantages in the cell operation bothin maintaining high current efliciencies over the entire life of theanode and the lesser attention required to the operation of the cell.Using the present method, the flow through the cell is maintainedindependent of the porosity of the diaphragm. Thus, as the diaphragmbecomes restricted and loses some of its permeability, the level withinthe anolyte compartment of the cell is controlled so that feed solutionin excess of that flowing through the diaphragm is circumvented aroundthe diaphragm and fed to the catholyte compartment. This is particularlyadvantageous in maintaining the current efficiency by keeping thesalt-caustic ratio of the catholyte liquor within the most preferredconcentrations commensurable with the most desirable currentefliciencies. Previously, increases in the concentration of caustic inthe catholyte compartment, due to the slower flow rate, reduced theanode current efficiency. The present method is also particularly usefulin high current density cells, that is, cells operating at about 1 to 5amperes per square inch of cathode surface. Also, the present inventionis particularly useful in chlorate production methods which utilizec'hlor-alkali diaphragm cells and an electrolyte which contains chlorateas well as chloride.

The invention will be described more particularly with reference to thedrawing which is a partial sectional view of a chlor-alkali diaphragmcell illustrating an apparatus particularly useful in the presentinvention.

A chlor-alkali cell is composed of an anode 12 and a cathode 14,separated by a porous diaphragm 16 to form an anolyte compartment 22 anda catholyte compartment 26. The diaphragm 16 is normally applied to theouter surface of a foraminous cathode. Electrolyte 18, which is normallya concentrated brine solution, is fed to the chlor-alkali cell 10through inlet into the anolyte compartment 22. The electrolyte level 24within the anolyte compartment is maintained above the cathode 14 sothat a constant hydrostatic pressure is applied against diaphragm 16 asthe electrolyte flows into the catholyte compartment 26. In thecatholyte compartment 26, a liquid level 28 is maintained by means of anoverflow (not shown) which draws cell liquor out of the catholytecompartment.

During the operation of the electrolytic cell, the porosity of thediaphragm decreases as impurities begin to restrict the flow ofelectrolyte from the anolyte compartment into the catholyte compartment.As the flow of electrolyte through the diaphragm lessens, theelectrolyte level 24 in the anolyte compartment 22 increases if the feedrate of electrolyte to the cell is maintained constant. Eventually, theflow of electrolyte to the cell is lessened or the excess brine willoverflow the sightglass or otherwise be expelled from the anolytecompartment and wasted.

In the present invention, an overflow means 30 is positioned within theanolyte compartment 22 to withdraw excess electrolyte from the anolytecompartment or alternatively, brine is fed in the desired amount ofabout 0.005 to about 1.5 and more preferably 0.01 to about 0.5 times theflow through the diaphragm directly to the catholyte compartment.Overflow means 30 can conveniently feed the excess liquor to hydrogengas withdrawal means 32 which is in communication with catholytecompartment 26. When overflow means 30 is connected to hydrogen gaswithdrawal means 32 or the like, gas trap 34 is preferably provided toeliminate the danger of hydrogen gas back feeding into the anolytecompartment 22 wherein it could mix with the chlorine gas to form anexplosive mixture. Overflow means 30 is preferably constructed in amanner whereby the electrolyte level 24 in anolyte compartment 22 can bereadily adjusted by raising or lowering the intake end 31 of overflowmeans 30. A convenient means of effecting such an adjustment is bycurving a section of overflow means 30 in the manner illustrated in thedrawing, thereby providing for a means of readily increasing ordecreasing the electrolyte level 24 by merely rotating overflow means 30to raise and lower intake end 31.

Alternatively, other means for withdrawing electrolyte from the anolytecompartment can be used with correspondingly efifective results. Forinstance, an internal overflow pipe of adjustable length can be placedto extend upwardly from the bottom of the catholyte compartment 26through the cathode 14 and upwardly into the anolyte compartment 22 toterminate with a liquid intake at the electrolyte level desired. Inanother embodiment, an orifice of adjustable size can be used to controlthe flow of electrolyte from the anode compartment through the diaphragminto the cathode compartment.

The chlor-alkali cell of the present invention is preferably operatedutilizing an alkali metal chloride brine, particularly sodium chloride.Other alkali metal chlorides such as lithium chloride, potassiumchloride, cesium chloride, rubidium chloride and the like are used toproduce the corresponding caustic material. Because of the plentifulsupply of sodium chloride and relatively low cost thereof, and thehigher demand for caustic soda, sodium chloride is normally the brinefeed material. Because of this, the invention will be described moreparticularly with reference to sodium chloride. However, when sodiumchloride is described as the alkali metal chloride, it is to beconsidered that other alkali metal chlorides can be substituted thereforwith correspondingly good results.

The present invention is operated by feeding a brine solution to theelectrolytic cell at a rate in excess of the rate at which the brineflows from the anolyte compartment through the diaphragm into thecatholyte compartment. Normally, this occurs after the cell has gonehigh level due to the restriction of the diaphragm, but it can also beproduced by using a higher than normal feed rate. The brine solution fedto the cell is a chloride solution containing about to 330 grams perliter of sodium chloride. Preferably, the brine is near the saturationpoint at the feed temperature. Thus, in a preferred method of operation,the aqueous brine solution contains about 260 to 330 grams per liter.

Under the decomposition voltage applied to the elec' trodes of theelectrolytic cell, chlorine is evolved at the anodes and hydrogen isevolved at the cathode along with the production of caustic. The sodiumhydroxide concentration in the catholyte compartment of a conventionaldiaphragm cell is about 9 to 12 percent sodium hydroxide. Thiscorresponds to about 110 to about grams per liter of sodium hydroxide.The exact concentration depends upon a number of variables, particularlythe rate of electrolyte flow through the cell and the current appliedwith respect to the electrolyte flow rate. Thus, at a constant current,a lesser flow rate through the diaphragm increases the sodium hydroxidecontent of the catholyte liquor. With an increased concentration ofhydroxyl ion and a decrease in salt concentration, the anode currentefliciency is adversely affected due to the back migration of hydroxylions into the anolyte compartment. Therefore, as an embodiment of thepresent invention, it is preferred to regulate the anolyte pH within acertain desirable pH range to produce the most desirable anode currentefliciency.

It has been found that the anode current efficiency is the highest at apH in the range of about 1 to 4. Therefore, the brine fed to the cell ispreferably acidified with hydrochloric acid in either gaseous or liquidform in an amount suflicient to provide the most desired acidity withinthe anolyte compartment. Normally, unacidified feed brine has a pH ofabout 9. The chlorine evolved from the electrolytic cell in the anodecompartment contributes to the reduction of the pH and therefore, thebrine feed solution need not necessarily be reduced in acidity to a pHof 4 prior to entering the anolyte compartment but rather acidificationto below a pH of about 6 to 7 is often sufiicient to result in the pHbeing within the desired 1 to 4 range during cell operation.

To compensate for the back migration of hydroxyl ions, and thecorresponding adverse affect on the anolyte pH, the present inventionretains and/or regulates the concentration of caustic in the catholytecompartment to within the most desirable range independent of theporosity of the diaphragm. The excess brine withdrawn from the anolytecompartment and added to the catholyte compartment dilutes the catholyteliquor and retains the salt-caustic ratio in the catholyte liquor towithin the most desirable range thereby retaining higher anode andcathode current efliciencies. Therefore, in accordance with the presentinvention the salt-caustic ratio in the normal production of chlorineand caustic is retained within the range of about 800 to 2,500, and morepreferably within the range of 1,200 to 1,700 wherein the salt causticratio is determined by the formula Grams per liter of salt Grams perliter of caustic 1000 favorable voltage drop. In addition to asbestos,certain inert organic materials can be used with correspondingly goodresults. Materials such as chlorinated polyvinyl chloride,polyvinylidene chloride, polyvinyl fluoride, polyvinylidene fluoride,polypropylene and the like in woven or fabric form can be used withcorrespondingly good results.

The following examples illustrate certain preferred embodiments of thepresent invention. Unless otherwise indicated, all parts and percentagesused herein are by weight and all temperatures are in degreescentigrade.

EXAMPLE 1 The present invention was operated utilizing a Hooker type S-lcell having a deposited asbestos diaphragm. The electrolytic cell hadbeen operating in the usual manner for 142 days on the same diaphragmwith initially new graphite anodes. As of the l42nd day, the diaphragmhad become restricted so that the original feed rate had been reduced byabout 50 percent. The current efliciency of the cell averaged 93.5percent. The catholyte liquor was comprised of 166 grams per liter ofsodium hydroxide and the salt caustic ratio was 1,078 and the chlorateconcentration was one pound per 1,000 pounds of caustic. These ratioswere obtained at a brine feed concentration of about 320 grams per literof sodium chloride. Normally, a cell operating in this manner would havehad the diaphragm replaced.

In accordance with the invention, the cell was modified by theinstallation of an overflow device in communication with'the anolytecompartment to withdraw excess electrolyte from the anolyte comparmentand pass it into the hydrogen gas withdrawal pipe and subsequently intothe catholyte chamber in accordance with the drawing. The flow rate ofbrine to the electrolytic cell was then increased to the normal flow ofabout 0.7 gallon per minute. Analysis of electrolysis products indicatedthat the current efficiency had increased to an 8 day average of 95.0percent and that the sodium hydroxide concentration in the catholytecompartment had been reduced to an average of 125 grams per liter. Also,a more desirable saltcaustic ratio of about 1,680 was obtained. Theincreased efliciency continued for the life of the anodes. In additionto the increased efiiciency and a more desirable salt-caustic ratio, therequirement for renewal of the diaphragm was eliminated. After 212 daysof operation, the sodium hydroxide concentration and salt-caustic ratioin the catholyte liquor was substantially unchanged.

EXAMPLE 2 The present invention was again operated utilizing a Hookertype S-l cell having a deposited asbestos diaphragm. The electrolyticcell had been operating for an extended period of time and the currentefficiency had dropped to 92.7 percent. The flow rate of feed brine tothe anolyte compartment had been reduced due to the lessening of thediaphragm porosity. The salt-caustic ratio was about 1,110 and thecaustic concentration averaged 154 grams per liter of sodium hydroxidein the catholyte liquor. The cell was modified in accordance with theinvention by the installation of an overflow device in communicationwith the catholyte compartment to thereby withdraw a portion of theelectrolyte from the anolyte compartment and passing it into thehydrogen gas withdrawal pipe from whence it passed into the catholytechamber of the electrolytic cell. The flow rate of the brine to theanolyte compartment in the electrolytic cell was then returned to thenormal flow rate of about 0.7 gallon per minute. Initially, about 10percent of this flow was withdrawn from the anolyte compartment andpassed into the catholyte compartment of the cell thereby circumventingthe diaphragm. On continued operation, the percentage of bypassingelectrolyte increased with the age of the diaphragm. Analysis of thecurrent efficiency for the next 14 days of cell operation averaged 95.3percent. The sodium hydroxide concentration in the catholyte liquor wasreduced to an average of about grams per liter and the salt-causticratio returned to a more favorable level of an average of about 1,500.The cell continued to operate at improved current efficiencies for theremaining life of the anodes. By this method, the current efficiency ofthe cell was returned to the desired operating range without therequirement for the renewal of the diaphragm and the associated expenseof renewal and shutdown time previously required.

In the same manner, the invention is operated in high current densitieschlor-alkali cells having current densities in the range of about 1 to 5amperes per square inch of cathode surface by feeding separate streamsof brine feed solution to both the anolyte and catholyte compartment.The catholyte feed rate is adjusted to provide the most preferred sodiumhydroxide concentration of about grams per liter.

While there have been described various embodiments of the presentinvention, the method and apparatus described are not intended to beunderstood as limiting the scope of the invention as it is realized thatchanges therein are possible. It is intended that each element recitedin any of the following claims is intended to be understood as referringto all equivalent elements for accomplishing substantially the sameresults in substantially the same or equivalent manner. It is intendedto cover the invention broadly in whatever form its principles may beutilized.

What is claimed is:

1. In a method for the production of chlorine and caustic using anelectrolytic diaphragm cell having an anolyte compartment and acatholyte compartment separated by a porous diaphragm, the improvementcomprising feeding brine solution to the anolyte compartment of saidcell while feeding a supplemental brine solution from the anolytecompartment through a hydrogen gas withdrawal means to the catholytecompartment of said cell in excess of the flow through the diaphragm.

2. The process of claim 1 wherein the first said brine solution is fedto the anolyte compartment of said cell at a rate in excess of the flowthrough the diaphragm, withdrawing the excess feed solution from theanolyte compartment, circumventing the diaphragm with said excess feedsolution and passing said excess solution to the catholyte compartmentof an electrolytic cell.

3. The process of claim 1 wherein the electrolyte fed to the anolytecompartment is a sodium chloride solution of a concentration of about120 to 330 grams per liter.

4. The process of claim 1 wherein the electrolyte solution in theanolyte compartment is maintained at a pH of about 1 to 4 by theaddition of HCl.

5. The process of claim 1 wherein the flow of electrolyte to thecatholyte compartment is in an amount of about 0.005 to about 1.5 timesthe flow of the electrolyte through the diaphragm.

6. A chlor-alkali diaphragm cell comprising an anode and a cathodeseparated by a porous diaphragm thereby forming an anolyte compartmentand a catholyte compartment, a hydrogen gas withdrawal means incommunication with said catholyte compartment, a liquid passage meanscommunicating between said anolyte compartment and said hydrogen gaswithdrawal means for the passage of anolyte liquor to the catholytecompartment without passing through said diaphragm.

7. The apparatus of claim 6 wherein the liquid passage means comprises atubular passage.

8. The apparatus of claim 7 wherein the tubular passage has a curvedportion which extends into the anolyte compartment.

References Cited UNITED STATES PATENTS Gaus 20498 Gibbs 204-98 Dow204-98 Heiskell et al. 20498 Kircher 204-98 10 Inove et al 20499 X Cox20498 Cooper 20498 Currey et al. 20498 8 FOREIGN PATENTS 3/1929 France.

OTHER REFERENCES Hooker Chemical Corp., Bulletin 20-A Hooker Type SCells, Niagara Falls, N.Y., 1959, 8 pgs.

Mantel], C. L., Electrochemical Engineering (4th Ed.) McGraw-Hill, NewYork, 1960, pp. v and 280-285. WINSTON A. DOUGLAS, Primary Examiner A.BEKELMAN, Assistant Examiner US. Cl. X.R. 204263, 266

1. IN A METHOD FOR THE PRODUCTION OF CHLORINE AND CAUSTIC USING ANELECTROLYTIC DIAPHRAGM CELL HAVING AN ANOLYTE COMPARTMENT AND ACATHOLYTE COMPARTMENT SEPARATED BY A POROUS DIAPHRAGM, THE IMPROVEMENTCOMPRISING FEEDING BRINE SOLUTION TO THE ANOLYTE COMPARTMENT OF SAIDCELL WHILE FEEDING A SUPPLEMENTAL BRINE SOLUTION FROM THE ANOLYTECOMPARTMENT THROUGH A HYDROGEN GAS WITHDRAWAL MEANS TO THE CATHOLYTECOMPARTMENT OF SAID CELL IN EXCESS OF THE FLOW THROUGH THE DIAPHRAGM. 6.A CHLOR-ALKALI DIAPHRAGM CELL COMPRISING AN ANODE AND A CATHODESEPARATED BY A POROUS DIAPHRAGM THEREBY FORMING AN ANOLYTE COMPARTMENTAND A CATHOLYTE COMPARTMENT, A HYDROGEN GAS WITHDRAWAL MEANS INCOMMUNICATION WITH SAID CATHOLYTE COMPARTMENT, A LIQUID PASSAGE MEANSCOMMUNICATING BETWEEN SAID ANOLYTE COMPARTMENT AND SAID HYDROGEN GASWITHDRAWAL MEANS FOR THE PASSAGE OF ANOLYTE LIQUOR TO THE CATHOLYTECOMPARTMENT WITHOUT PASSING THROUGH SAID DIAPHRAGM.